1 /*
2 * Copyright (c) 1998, 2008, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 // The SplitWord construct allows us to colocate the contention queue
26 // (cxq) with the lock-byte. The queue elements are ParkEvents, which are
27 // always aligned on 256-byte addresses - the least significant byte of
28 // a ParkEvent is always 0. Colocating the lock-byte with the queue
29 // allows us to easily avoid what would otherwise be a race in lock()
30 // if we were to use two completely separate fields for the contention queue
31 // and the lock indicator. Specifically, colocation renders us immune
32 // from the race where a thread might enqueue itself in the lock() slow-path
33 // immediately after the lock holder drops the outer lock in the unlock()
34 // fast-path.
35 //
36 // Colocation allows us to use a fast-path unlock() form that uses
37 // A MEMBAR instead of a CAS. MEMBAR has lower local latency than CAS
38 // on many platforms.
39 //
40 // See:
41 // + http://blogs.sun.com/dave/entry/biased_locking_in_hotspot
42 // + http://blogs.sun.com/dave/resource/synchronization-public2.pdf
43 //
44 // Note that we're *not* using word-tearing the classic sense.
45 // The lock() fast-path will CAS the lockword and the unlock()
46 // fast-path will store into the lock-byte colocated within the lockword.
47 // We depend on the fact that all our reference platforms have
48 // coherent and atomic byte accesses. More precisely, byte stores
49 // interoperate in a safe, sane, and expected manner with respect to
50 // CAS, ST and LDs to the full-word containing the byte.
51 // If you're porting HotSpot to a platform where that isn't the case
52 // then you'll want change the unlock() fast path from:
53 // STB;MEMBAR #storeload; LDN
54 // to a full-word CAS of the lockword.
55
56
57 union SplitWord { // full-word with separately addressable LSB
58 volatile intptr_t FullWord ;
59 volatile void * Address ;
60 volatile jbyte Bytes [sizeof(intptr_t)] ;
61 } ;
62
63 // Endian-ness ... index of least-significant byte in SplitWord.Bytes[]
64 #ifdef VM_LITTLE_ENDIAN
65 #define _LSBINDEX 0
66 #else
67 #define _LSBINDEX (sizeof(intptr_t)-1)
68 #endif
69
70 class ParkEvent ;
71
72 // See orderAccess.hpp. We assume throughout the VM that mutex lock and
73 // try_lock do fence-lock-acquire, and that unlock does a release-unlock,
74 // *in that order*. If their implementations change such that these
75 // assumptions are violated, a whole lot of code will break.
76
77 // The default length of monitor name is chosen to be 64 to avoid false sharing.
78 static const int MONITOR_NAME_LEN = 64;
79
80 class Monitor : public CHeapObj {
81
82 public:
83 // A special lock: Is a lock where you are guaranteed not to block while you are
84 // holding it, i.e., no vm operation can happen, taking other locks, etc.
85 // NOTE: It is critical that the rank 'special' be the lowest (earliest)
86 // (except for "event"?) for the deadlock dection to work correctly.
87 // The rank native is only for use in Mutex's created by JVM_RawMonitorCreate,
88 // which being external to the VM are not subject to deadlock detection.
89 // The rank safepoint is used only for synchronization in reaching a
90 // safepoint and leaving a safepoint. It is only used for the Safepoint_lock
91 // currently. While at a safepoint no mutexes of rank safepoint are held
92 // by any thread.
93 // The rank named "leaf" is probably historical (and should
94 // be changed) -- mutexes of this rank aren't really leaf mutexes
95 // at all.
96 enum lock_types {
97 event,
98 special,
99 suspend_resume,
100 leaf = suspend_resume + 2,
101 safepoint = leaf + 10,
102 barrier = safepoint + 1,
103 nonleaf = barrier + 1,
104 max_nonleaf = nonleaf + 900,
105 native = max_nonleaf + 1
106 };
107
108 // The WaitSet and EntryList linked lists are composed of ParkEvents.
109 // I use ParkEvent instead of threads as ParkEvents are immortal and
110 // type-stable, meaning we can safely unpark() a possibly stale
111 // list element in the unlock()-path.
112
113 protected: // Monitor-Mutex metadata
114 SplitWord _LockWord ; // Contention queue (cxq) colocated with Lock-byte
115 enum LockWordBits { _LBIT=1 } ;
116 Thread * volatile _owner; // The owner of the lock
117 // Consider sequestering _owner on its own $line
118 // to aid future synchronization mechanisms.
119 ParkEvent * volatile _EntryList ; // List of threads waiting for entry
120 ParkEvent * volatile _OnDeck ; // heir-presumptive
121 volatile intptr_t _WaitLock [1] ; // Protects _WaitSet
122 ParkEvent * volatile _WaitSet ; // LL of ParkEvents
123 volatile bool _snuck; // Used for sneaky locking (evil).
124 int NotifyCount ; // diagnostic assist
125 char _name[MONITOR_NAME_LEN]; // Name of mutex
126
127 // Debugging fields for naming, deadlock detection, etc. (some only used in debug mode)
128 #ifndef PRODUCT
129 bool _allow_vm_block;
130 debug_only(int _rank;) // rank (to avoid/detect potential deadlocks)
131 debug_only(Monitor * _next;) // Used by a Thread to link up owned locks
132 debug_only(Thread* _last_owner;) // the last thread to own the lock
133 debug_only(static bool contains(Monitor * locks, Monitor * lock);)
134 debug_only(static Monitor * get_least_ranked_lock(Monitor * locks);)
135 debug_only(Monitor * get_least_ranked_lock_besides_this(Monitor * locks);)
136 #endif
137
138 void set_owner_implementation(Thread* owner) PRODUCT_RETURN;
139 void check_prelock_state (Thread* thread) PRODUCT_RETURN;
140 void check_block_state (Thread* thread) PRODUCT_RETURN;
141
142 // platform-dependent support code can go here (in os_<os_family>.cpp)
143 public:
144 enum {
145 _no_safepoint_check_flag = true,
146 _allow_vm_block_flag = true,
147 _as_suspend_equivalent_flag = true
148 };
149
150 enum WaitResults {
151 CONDVAR_EVENT, // Wait returned because of condition variable notification
152 INTERRUPT_EVENT, // Wait returned because waiting thread was interrupted
153 NUMBER_WAIT_RESULTS
154 };
155
156 private:
157 int TrySpin (Thread * Self) ;
158 int TryLock () ;
159 int TryFast () ;
160 int AcquireOrPush (ParkEvent * ev) ;
161 void IUnlock (bool RelaxAssert) ;
162 void ILock (Thread * Self) ;
163 int IWait (Thread * Self, jlong timo);
164 int ILocked () ;
165
166 protected:
167 static void ClearMonitor (Monitor * m, const char* name = NULL) ;
168 Monitor() ;
169
170 public:
171 Monitor(int rank, const char *name, bool allow_vm_block=false);
172 ~Monitor();
173
174 // Wait until monitor is notified (or times out).
175 // Defaults are to make safepoint checks, wait time is forever (i.e.,
176 // zero), and not a suspend-equivalent condition. Returns true if wait
177 // times out; otherwise returns false.
178 bool wait(bool no_safepoint_check = !_no_safepoint_check_flag,
179 long timeout = 0,
180 bool as_suspend_equivalent = !_as_suspend_equivalent_flag);
181 bool notify();
182 bool notify_all();
183
184
185 void lock(); // prints out warning if VM thread blocks
186 void lock(Thread *thread); // overloaded with current thread
187 void unlock();
188 bool is_locked() const { return _owner != NULL; }
189
190 bool try_lock(); // Like lock(), but unblocking. It returns false instead
191
192 // Lock without safepoint check. Should ONLY be used by safepoint code and other code
193 // that is guaranteed not to block while running inside the VM.
194 void lock_without_safepoint_check();
195 void lock_without_safepoint_check (Thread * Self) ;
196
197 // Current owner - not not MT-safe. Can only be used to guarantee that
198 // the current running thread owns the lock
199 Thread* owner() const { return _owner; }
200 bool owned_by_self() const;
201
202 // Support for JVM_RawMonitorEnter & JVM_RawMonitorExit. These can be called by
203 // non-Java thread. (We should really have a RawMonitor abstraction)
204 void jvm_raw_lock();
205 void jvm_raw_unlock();
206 const char *name() const { return _name; }
207
208 void print_on_error(outputStream* st) const;
209
210 #ifndef PRODUCT
211 void print_on(outputStream* st) const;
212 void print() const { print_on(tty); }
213 debug_only(int rank() const { return _rank; })
214 bool allow_vm_block() { return _allow_vm_block; }
215
216 debug_only(Monitor *next() const { return _next; })
217 debug_only(void set_next(Monitor *next) { _next = next; })
218 #endif
219
220 void set_owner(Thread* owner) {
221 #ifndef PRODUCT
222 set_owner_implementation(owner);
223 debug_only(void verify_Monitor(Thread* thr));
224 #else
225 _owner = owner;
226 #endif
227 }
228
229 };
230
231 // Normally we'd expect Monitor to extend Mutex in the sense that a monitor
232 // constructed from pthreads primitives might extend a mutex by adding
233 // a condvar and some extra metadata. In fact this was the case until J2SE7.
234 //
235 // Currently, however, the base object is a monitor. Monitor contains all the
236 // logic for wait(), notify(), etc. Mutex extends monitor and restricts the
237 // visiblity of wait(), notify(), and notify_all().
238 //
239 // Another viable alternative would have been to have Monitor extend Mutex and
240 // implement all the normal mutex and wait()-notify() logic in Mutex base class.
241 // The wait()-notify() facility would be exposed via special protected member functions
242 // (e.g., _Wait() and _Notify()) in Mutex. Monitor would extend Mutex and expose wait()
243 // as a call to _Wait(). That is, the public wait() would be a wrapper for the protected
244 // _Wait().
245 //
246 // An even better alternative is to simply eliminate Mutex:: and use Monitor:: instead.
247 // After all, monitors are sufficient for Java-level synchronization. At one point in time
248 // there may have been some benefit to having distinct mutexes and monitors, but that time
249 // has past.
250 //
251 // The Mutex/Monitor design parallels that of Java-monitors, being based on
252 // thread-specific park-unpark platform-specific primitives.
253
254
255 class Mutex : public Monitor { // degenerate Monitor
256 public:
257 Mutex (int rank, const char *name, bool allow_vm_block=false);
258 ~Mutex () ;
259 private:
260 bool notify () { ShouldNotReachHere(); return false; }
261 bool notify_all() { ShouldNotReachHere(); return false; }
262 bool wait (bool no_safepoint_check, long timeout, bool as_suspend_equivalent) {
263 ShouldNotReachHere() ;
264 return false ;
265 }
266 };
267
268 /*
269 * Per-thread blocking support for JSR166. See the Java-level
270 * Documentation for rationale. Basically, park acts like wait, unpark
271 * like notify.
272 *
273 * 6271289 --
274 * To avoid errors where an os thread expires but the JavaThread still
275 * exists, Parkers are immortal (type-stable) and are recycled across
276 * new threads. This parallels the ParkEvent implementation.
277 * Because park-unpark allow spurious wakeups it is harmless if an
278 * unpark call unparks a new thread using the old Parker reference.
279 *
280 * In the future we'll want to think about eliminating Parker and using
281 * ParkEvent instead. There's considerable duplication between the two
282 * services.
283 *
284 */
285
286 class Parker : public os::PlatformParker {
287 private:
288 volatile int _counter ;
289 Parker * FreeNext ;
290 JavaThread * AssociatedWith ; // Current association
291
292 public:
293 Parker() : PlatformParker() {
294 _counter = 0 ;
295 FreeNext = NULL ;
296 AssociatedWith = NULL ;
297 }
298 protected:
299 ~Parker() { ShouldNotReachHere(); }
300 public:
301 // For simplicity of interface with Java, all forms of park (indefinite,
302 // relative, and absolute) are multiplexed into one call.
303 void park(bool isAbsolute, jlong time);
304 void unpark();
305
306 // Lifecycle operators
307 static Parker * Allocate (JavaThread * t) ;
308 static void Release (Parker * e) ;
309 private:
310 static Parker * volatile FreeList ;
311 static volatile int ListLock ;
312 };